1. Field of the Invention
The present invention relates to an insulated gate type semiconductor device using a crystalline semiconductor film. In particular, it relates to a semiconductor device having a CMOS structure using silicon film for the semiconductor film.
2. Description of the Related Art
Recently, active development is made on the CMOS technology using insulated gate type transistor. However, as is disclosed in Japanese Unexamined Patent Publication Hei 4-206971 and in Japanese Unexamined Patent Publication Hei 4-286339, the electric properties of an N-type transistor using a crystalline silicon film for the active layer tend to shift to the direction of depression (negative side), whereas those of a P-type transistor tend to shift to the enhancement direction (negative side).
More specifically, the threshold voltage of an N-type transistor (Vth,n) tends to lower, whereas that of a P-type transistor (Vth,p) tends to elevate. This difference in threshold voltage makes it difficult to determine the voltage of the power source.
To overcome the problem above, a variety of methods have been proposed for controlling the threshold value. For instance, in the field of ICs, in general, the threshold value is controlled by the carrier density; the method comprises changing the impurity concentration of the well or of the channel region. Also proposed is a method comprising changing the impurity concentration of the gate electrode made from a polysilicon film, wherein the threshold value is controlled by the difference in work function (e.g., in Dual gate CMOS structure, etc.)
The basic constitution of the methods above for controlling the threshold value is to control the threshold voltage by the concentration of the added impurity. However, controlling the impurity concentration has a disadvantage in that it has difficulties from the viewpoint of achieving uniformity and reproducibility, and many factors remain uncertain in constantly controlling the threshold value to a constant value.
Furthermore, with increasing fineness in semiconductor devices, the influence on carriers cast by the impurity elements that are present in the channel forming region becomes non-negligible. Thus, the method of controlling the threshold value by means of impurity concentration sometimes lowers the operation speed of the semiconductor device.